$startTime = array_sum(explode(' ', microtime()));
//code
$endTime = array_sum(explode(' ', microtime()));
echo 'This page took approximately '.round($endTime - $startTime, 7).'seconds to load.';

Tutorial

This code calculates the time it takes to interpret and run the PHP code.

$startTime = array_sum(explode(' ', microtime()));

The time that the code starts is recorded. This is done by calling microtime which returns 2 parts (the current time in seconds and milliseconds), for better accuracy we sum both parts.

//code

Insert the script here that will be timed.

$endTime = array_sum(explode(' ', microtime()));
echo 'This page took approximately '.round($endTime - $startTime, 7).'seconds to load.';

Finally the time after the code has been interpreted is stored and the difference between start time and end time is printed to the user.